This section provides background information related to the present disclosure which is not necessarily prior art.
EP 0 410 271 B1 describes a radial fan for conveying a gaseous medium in a device with high flow resistance, in particular a burner in a gas boiler. These types of burner have a relatively high flow resistance of in the region of 200 pascals or over. The medium to be conveyed can be air or a combustible gas/air mixture. The fan and the fan impeller of the same must therefore be designed to have a steep pressure/volume flow characteristic curve. This means that pressure changes should only be associated with small changes to the volume flow. For this, the known fan has a fan impeller, the diameter of which is more than ten times its flow discharge width. This is a closed wheel, covered on both sides, with a substantially flat lower portion having a central hub (first covering disc), a plurality of blades bent rearwards, and each being in the shape of a segment of a circle, and a circular, flat, plate-shaped cover (second covering disc) with a central, circular inlet opening.
Documents DE 41 41 359 A1, CH 301 116 and DE 102 04 037 A1 respectively describe similar fan impellers which are also closed, according to the first two documents shortened intermediary blades being provided.
These types of closed radial fan impeller, covered on both sides, are relatively difficult and expensive to produce.
A further radial fan is known from WO 02/45862 A2 which, however, has a fan impeller with a large axial length and flow discharge width in comparison with the diameter such that this fan is not suited, or is less well suited to the preferred application as a gas burner fan. This known fan impeller has blades which are all of the same length and is designed to be axially open on the hub side.